1. Field of the Invention
The present invention relates to lead-free and preferably also arsenic-free lanthanum heavy flint glass with a refractive index nd of 1.84≦nd≦1.96 and an Abbé number νd of 27≦νd≦36, and to the uses of this glass.
2. Description of the Related Art
Conventional optical glass in the lanthanum-heavy flint range with a low Abbé number for the application areas of imaging, projection, telecommunications, optical communication technology and laser technology generally contains PbO, in order to achieve the desired optical properties, i.e. a refractive index nd of 1.84≦nd≦1.96 and, in particular, the high dispersion, i.e. a low Abbé number νd of 27≦νd≦36. However, PbO reduces the chemical resistance of these types of glass. Furthermore, materials with high chemical resistance are becoming increasingly important for use in high added value products. Moreover, As2O3 is often used as a refining agent. Since in recent years the glass components PbO and As2O3 have been regarded as environmentally harmful, most manufacturers of optical instruments and products tend preferably to use lead-free and arsenic-free glass.
Known lead-free glass compositions with a high refractive index and a low Abbé number generally contain extremely large quantities of TiO2 in a silicate matrix, which leads to glass, which is very highly susceptible to crystallization and extremely difficult to process.
In addition, in terms of melting technology there have recently been reports of increased demand for “short” glass, i.e. for glass whose viscosity varies extremely strongly with the temperature. During the manufacturing process, this characteristic has the advantage that the hot-shaping times, i.e. the mold-closed times, can be reduced. This firstly increases throughput and secondly protects the mold material, with the result that total production costs can be reduced considerably. Also, the more rapid cooling which is thereby made possible also allows glass with a relatively strong tendency to crystallize to be processed, i.e. correspondingly longer glass, and means that preliminary nucleation, which could cause problems in subsequent secondary hot-forming steps, is avoided.
Therefore, a composition range for short optical glass, which allows the desired optical properties with regard to nd and νd to be achieved, even without use of PbO and As2O3, and furthermore with a reduced TiO2 content, would be advantageous.
However glass with a similar optical position or comparable chemical composition, which has hitherto been disclosed in the prior art, has serious drawbacks.
For example, DE 691 356 (Eastman Kodak) describes silicate-free lanthanum borate glass for achieving a similar optical position, namely a very high refractive index with an extremely low dispersion (i.e. a high Abbé number), but this glass is at very great risk of crystallizing. Moreover, to stabilize this glass it is preferable to introduce very expensive tantalum oxide (or optionally highly toxic thorium oxide or expensive tungsten oxide) as crystallization inhibitor. Furthermore, when SiO2 is used without the addition of alkali metal oxides, opacification phenomena have been described, and consequently the use of silicate is obligatorily linked to the addition of alkali metals.
In a corresponding way to the document discussed above, JP 78-004023 A (Ohara) also describes glass with an extremely low dispersion and very high refractive index. The glass described differs from the glass described in DE 691 356 through the obligatory requirement that hafnium must be used to stabilize the lanthanum borate matrix. However, on account of the difficulty of purifying raw materials, this component is extremely expensive. Moreover, it has a very high melting point (melting point 3050° C.) compared to its more usual homolog TiO2 (melting point 1560° C.) and ZrO2 (melting point 1700° C.) with comparable physico-chemical characteristics. The result of this is that the melting process is made considerably more difficult by additions of hafnium oxide.
JP 84-050048 A (Ohara) describes silicate-containing (>8% by weight) lanthanum borate glasses, without alkali metal oxides necessarily being added. Since the solubility of lanthanum oxide in a borosilicate matrix is significantly worse than in a pure borate matrix, the maximum amount of lanthanum, which can be used in this glass, is limited. Therefore, the result is either glass whose refractive indices are lower on account of a lower La2O3 content or whose crystallization resistance is adversely affected by increasing the refractive index with additional oxides.
DE 31 38 137 (SCHOTT GLAS) relates to glass with an extremely low dispersion combined, at the same time, with a high refractive index. The stabilizing effect with respect to the tendency to crystallize caused by the use of silicate without alkali metals is achieved by adding large amounts of Nb2O5 (≧15% by weight). The glass described in this patent differs from that of DE 691 356 by the use of silicate in the absence of alkali metals and tantalum. However, since Nb2O5 is a relatively expensive component, such high levels of Nb2O5 are not economical.
DE 10 47 994 (Izumitani, et al) deals with lanthanum borate glass with a particularly high borate content (≧20% by weight). Although this has a positive influence on the solubility of lanthanum and therefore reduces the tendency to crystallize when silicate is used at the same time, the maximum refractive index, which can be reached, is reduced to below 1.87. Therefore, the glass described in this document tends to be designed for moderate optical applications with good chemical resistance, high grinding hardness, i.e. good machining properties, and low crystallization, rather than to achieve a high refractive index combined with a low dispersion.